Water in Food Flashcards
Describe the role of water in food. [6]
- Ubiquitous!
- Media for temperature
- Important in flavour/taste
- Important in safety
- Solvent, reaction medium and reactant
- Carrier of nutrients and waste products
- Lubricant, plasticizer (water-soluble materials are physically transformed), stabilizer
What is a plasticizer?
Materials that are water-soluble are physically transformed (usually using differences in solubility)
Describe water in food systems. [4]
- Continuous phase (aqueous) in dispersions and solutions
- Water entrapment in 3D continuous phases
- Entrapment in cells
- Attraction to hydrophilic compounds
What is free water? [3]
- Lightly entrapped
- Acts as dispersing agent and solvent
- Can be removed by drying
What is adsorbed water? [3]
- Associates in layer through intermolecular hydrogen bonds around hydrophilic food particles.
- Water is held tightly in cell walls (protoplasm) and is bound to proteins
- Not free; not heavily bound
What is bound water? [4]
- Unavailable as a solvent
- Tightly bound, remains unfrozen below 0C.
- ~8-10% of total water in animal tissues is bound
- Unfreezable water is based on protein content; varies
How does water influence the physical state of materials?
Dehydration is the rapid removal of water and transformation of the solutes back to an amorphous state, leaving large internal surface areas.
Water is a plasticizer whereby materials that are water-soluble are physically transformed.
What are the three forms of water in food?
Free water; dispersing agent & solvent; can be removed by drying
Adsorbed water; intermolecular hydrogen bonds form around hydrophilic molecules (i.e., hydration shell); or capillary entrapment; tightly held water unavailable for ice formation or microbe use
Bound water; remains unfrozen below 0C; bound to monolayer; unavailable for freezing
Describe the structure of a water molecule.
- Water molecule is composed of a single atom of oxygen covalently bound to two atoms of hydrogen.
- Unequal distribution of positive and negative charge makes the water molecule bipolar.
- The charges cause a bond angle of 104.5; giving approximate tetrahedral structure.
- Each hydrogen atom shares a pair of electrons with the oxygen atom (= covalent bond)
- Water molecules are held together by hydrogen bonding.
Describe the water molecule in ice.
- Each water molecule is associated with 4 other water molecules in a hexagonal fashion, with an open network structure
- The volume of water expands as temperature drops and as heat is withdrawn
- If you apply heat, ice melts, due to molecular motion that puts stress on the strength of the hydrogen bonds (affects density and viscosity of water)
Describe the 3 phases of water.
What happens to entropy when solutes are dissolved in water? What does this cause? [2]
Entropy decreases since water is oriented with respect to solute.
Water molecules are less free to escape from liquid in vapour phase; thus, vapour pressure is lowered.
This causes (1) depression in freezing point and (2) elevation in boiling point.
Compare the physical properties of water and ice.
Heat capacity of water is greater than ice.
What is the difference between latent heat and sensible heat.
Latent heat: energy needed to induce a phase change without changing temperature
Sensible heat: change in temperature of a substance without phase change
How is freeze drying accomplished?
A vacuum reduces pressure then heated slightly resulting in dehydration by sublimation
Discuss supercooled liquid water.
- Can be as cold as -48C.
- Ice from liquid water requires a ‘seed crystal’ to become the nucleus around which other crystals form
- Hard to achieve in pure water with no contaminants
How many crystalline polymorphic structures can ice form?
8 crystalline
Non-crystalline structures also exist
Only relevant polymorph is Ice-Ih (ordinary hexagonal ice)
Ice holds shape as atoms or molecules are not free to move about but merely vibrate in fixed crystal lattices.
Describe the initial events of ice crystallization. [4]
- The first thermal event that occurs is super-cooling below the freezing point before the induction of crystallization.
- This is a non-equilibrium, metastable state which is analogous to an activation energy necessary to initiate nucleation.
- Pure water can be under-cooled by several degrees before nucleation
- Water, with a tight tetrahedral structure, crystallizes to open hexagonal system.
What are the events of ice crystal formation? [4]
- Undercooling
- Nucleation (generates latent heat of crystallization)
- Crystal growth
- Sensible heat
Describe ice formation in a sugar solution as compared to water.
Faster nucleation in presence of sugar; latent heat of crystallization is not as great because it starts to freeze and crystallize as a faster rate; freezing point depression to Tf.
Solutes depress the freezing point of water because they disrupt the hydrogen bonding that occurs between water as it begins to freeze into ice; solutes lower free energy of the system, therefore facilitating faster nucleation and enhanced crystallization.
Sugar solution doesn’t need to undercool as far, nucleation is faster, and nuclei that are formed are preserved resulting in better quality ice. Results in smaller, more homogenous, and greater in number (= more stable ice phase)
How is freezing time defined?
Onset of nucleation to end of crystal growth
Describe how freezing can affect the structure of foods.
Disruption of cell walls and membranes, for example.
Why is faster freezing preferable?
Produces small ice crystals; greater nucleation (results in more uniform/dense ice crystals and better organoleptic properties)
Minimizes concentration effects by increasing the time solutes contact other food constituents during transition from unfrozen to frozen.
Describe water interactions with ions and ionic groups that have polarizing power/electrical field strength.
Water’s tetrahedral arrangement is disrupted.
Entropy decreases.
Describe water interactions with nonpolar groups.
Leads to ‘hydrophobic hydration’ due to hydrophobic effect.
Describe water interactions with neutral groups. (hydrophilic and nonionic)
Hydrogen bonding occurs with hydroxyl, amino, carbonyl, amide, and imino groups; may lead to ‘water bridges’
Why does sucrose dissolve more slowly than salt?
The van der waals interactions that hold sucrose molecules together are not as strong as the ionic bonding of salt.
NaCl crystals are made of Na and Cl ions, which are charged entities. Water is also a polar molecule, and multiple water molecules can stabilize ions by forming a shield of sorts.
A crystal of sugar, on the other hand, is of sugar molecules, which are single, non charged, but polar units.
Simply put, NaCl(aq) is a much more tightly held state with water than sugar(aq), because ion water bonds are stronger and easier to form the sugar’s dipole bonds.
Describe the importance of the hydrophobic effect for protein spatial configuration and stability.
A nonpolar moiety imposes an increase in order of water; water forces nonpolar groups to aggregate; facilitated by release of water from low-energy hydration shells to high-energy free state.
What is water activity?
Quantifies water available for microbial or enzymatic activity.
The tendency of water molecules to escape the food from liquid to vapour.
Aw = p/po
p=partial vapour pressure of water in food
po = partial vapour pressure of pure water at similar temperature
Why is water activity important for product formulation?
For reasons like:
- Inhibition of microbial growth
- Shelf-life extension
- Moisture migration inhibition (e.g., from raisins to bran; higher aw to lower aw)
What is Raoult’s law?
Relationship between concentration of solutes and vapour pressure.
An ideal solute; relative lowering of vapour pressure is equal to the mole fraction of solute.
Estimate aw of 10% mass of sucrose solution.
MW (water) = 18 kg/kgmol
MW (sucrose) = 342 kg/kgmol
mole fraction = mass fraction / molecular weight
mole fraction of water = 90/18
mole fraction of sucrose = 10/342
aw = moles water / (moles water + solute)
aw = 0.99
What is hysteresis? What is it an indicator of? What is it defined by?
The difference between desorption and adsorption isotherms.
Indicator of food stability; defined by chemical composition of food as well as T and P.
What is a moisture sorption isotherm?
Water activity relates to the moisture content of the food and can be measured using a moisture sorption isotherm.
What is the BET-point?
The amount of water needed to form the monolayer.
The point where type I water becomes type II.
Describe type I water.
Bound water
No chemical reactions can occur; no freezing can occur
Strongly bound monolayer; not sufficient to coat solutes
Water is adsorbed on the surface of macromolecular colloids
Water of hydration
Forces include hydrogen bonds and van der waals forces
Describe type I water.
Bound water
No chemical reactions can occur; no freezing can occur
Strongly bound monolayer; not sufficient to coat solutes
Water is adsorbed on the surface of macromolecular colloids
Water of hydration
Forces include hydrogen bonds and van der waals forces
Describe type II water.
Monolayer Moisture content
Restricted mobility
Less strongly bound water layers and capillary adsorbed water
Monolayer is adsorbed
First multilayers are added
Describe type III water.
Solvent and free water
Freezable; free for use by microbial life
Water fills intergranular spaces and spaces in pores
Mobile dispersing agent and solvent
Food in this zone requires refrigeration
More water than binding sites
Define:
Sorption
Adsorption
Desorption
Sorption: process wherein food solids reversibly combine with water molecules
Adsorption: humidification
Desorption: drying
How does water activity relate to temperature at constant moisture content?
Food water activity increases with increasing temperature (for any given moisture content)
Why is water content not a reliable predictor of safe and stable food systems?
Moisture content can vary from product to product and does not inform on ‘available’ water.
Describe the effect of water content on water activity. What forces control this? What reactions are facilitated?
Water activity will be higher when a certain water content is achieved by absorption of water from a dry food.
Controlled by three forces: (1) osmotic, (2) matrix, and (3) capillary.
This facilitates many reactions:
non enzymatic browning
fat oxidation
protein denaturation
starch retrogradation
vitamin degradation
enzymatic reactions
starch gelatinization
Describe the effect of solute concentration on water activity.
Higher solute concentration decreases water activity.
Very solute specific in terms of effect.
Discuss water activity and the stability of food.
- Higher water activity leads to lower stability
- Lipid oxidation reaction rate is greatest at lower aw (and higher aw)
- aw>0.45 leads to increased reaction rates for non-enzymatic browning, water-soluble nutrient loss, enzymatic activity, mold, yeast, and bacterial growth
- Higher aw solubilizes substrates required for chemical and enzymatic activity
Describe water activity and texture.
High aw foods = juicy, tender, chewy (decrease leads to toughness/staleness)
Low aw foods = crunchy (increase leads to soggy)
Discuss MR, vitamin B1 degradation, and microbial growth in regards to water activity.
Curves for MR, vitamin B1 degradation and microbial growth exhibit maximal rates at intermediate to high aw
Why are peanuts salted and roasted
Salted to prevent rancidity of the oils; desorption is 3-6x faster in peanuts; catalysts are no longer soluble; toasted to denature enzymes.
How may water activity be reduced?
Drying
Evaporating
Increasing solute concentration
Freezing (as fast as possible to promote small ice crystals)
Kill animals quickly since anaerobic conditions release water from proteins
Describe how water activity predicts shelf-life.
Food packaging considerations: the rate of exchange of moisture through a package barrier can change the water activity of the food and influence its shelf life.
Phosphates are added to meat to entrap water and prevent water loss, for example.
What is moisture migration?
Multi-component products; different components = different energy status of the water = driving force for moisture migration to come to an equilibrium
Moisture migrates from high aw to low aw.
What water activity will microbial proliferation occur?
aw > 0.5
Pathogenic bacteria have the highest aw requirements.
Discuss water in oil.
Even oil is capable of holding some water; max amount = saturation point (further water separates and forms distinct layer)
Water activity in oil can be measured; takes a long time to equilibrate
How is water activity measured?
Capcitance hygrometer: consists of two charged plates separated by a dielectric membrane which when water is absorbed, a change in capacity to hold charge increases and the capacitance is measured.
Roughly proportional to the water activity as determined by sensor-specific calibration.
Requires equilibrium with vapour pressure of water in the sample with vapour pressure of the headspace.
What are potentially hazardous foods?
Need to know pH and aw to ensure safety of a product.
PHF require time/temperature controls for safety to limit pathogenic organism growth or toxin formation.
Non-pathogenic foods have aw = 0.85 below aw for S.aureus, and pH = 4.6 below pH for C.botulinum growth and toxin production.
Explain the lipid oxidation curve.
- At very low water activity the reducing capacity of water does not exist; lipids oxidize in an auto-oxidation propagation reaction
- As water activity increases, neutralize the peroxy-radicals that are produced, less lipids are oxidized
- At higher water activity, enzymes that accelerate the reaction are solubilized
- At extremely high water activity, enzymes and substrates are diluted.
